A lens carrier of this kind is known generically from EP 1 094 348 B1.
Low-stress retention of optical elements is a core problem in the development of high-power UV objectives and DUV objectives. By low-stress retention is meant that the optical element undergoes as little deformation as possible by the mount itself and that dynamic and thermal loads do not lead to any tensions or to misalignment of the optical element.
A number of arrangements are known from the art for solving this problem, in which an optical element such as a lens is held in a mount by elastic means.
For example, an arrangement is known from German Patent Application DE 28 46 241 A1 in which a lens which is radially guided in its mount is pressed axially against a fixed support by at least two small plates overlapping the edge area of the lens. The small plates are made of an elastic material and are held by positive engagement in the mount such that they contact the lens accompanied by preloading.
A thermal expansion of the lens in axial direction can be compensated by the axially elastic support of the lens. Also, dynamic loads can be damped. In order also to enable a radial thermal expansion of the lens without radial tensions occurring in the lens which can lead to dramatic impairments in optical imaging, there should be a sufficient looseness of fit depending on the different thermal expansion coefficients of the mount and lens. Within the framework of this fit clearance, however, misalignments may come about under the influence of radial dynamic loads.
An axially elastic holder for optical components in a mount is also described in German Utility Model G 86 25 896 U1. The elastic holding element is formed here as a closed ring which contacts the mount in the edge area of the lens on the one hand and in a groove provided for this purpose on the other hand.
This solution is disadvantageous in that dynamic loads are transmitted to the lens via the radially rigid ring virtually without damping.
Just as in the solution outlined above, the spring forces act to hold the lens on the optically active surface. Because the amount of spring force changes depending on the thermal expansion of the lens, there are changed stress states in the lens which cannot satisfy the high requirements for imaging quality, particularly in high-power objectives.
DE 196 32 267 A1 discloses a lens carrier in which lock lugs formed at the lens mount contact an engagement groove formed at the lens (also cemented lens group) in order to hold the lens in the mount through frictional engagement. The position of the lens inside the mount is determined in radial direction by a clearance fit between an inner circumferential surface of the lens mount and an outer circumferential surface of the lens and in axial direction by the pressing of the edge area of an end face of the lens against a retaining projection of the mount.
The mount is neither thermally nor mechanically stable. With differing expansion and contraction of the lens and lens mount due to a thermal load, there is compulsorily a relative displacement of the lens with respect to the mount and, therefore, friction particularly where the retaining projection contacts the end face of the lens. As a result of the stick-slip brought about by the friction, the displacement takes place in an indefinite manner and the lens does not return to its initial position. In high-power objectives, a misalignment such as this can already considerably reduce imaging quality. It is also disadvantageous that the lens is pressed against along its optically active end surface, which can lead to deformation of the lens.
A holder which is elastic in radial direction is known from German Patent Application DE 35 21 640 A1. It is formed of at least three ribs of a highly elastic material, advantageously silicone rubber, which are arranged, preferably so as to be uniformly distributed, at the cylindrical circumferential surface of the lens. These ribs are compressed when the lens is inserted into the mount. As a result of the only slight indirect contact surface between the mount and the lens at the circumferential surface of the lens, the mount and lens can expand differently in axial direction unimpeded.
The centering of the lens in its mount is always ensured regardless of thermal and dynamic loads. It is also advantageous that the acting forces act at the circumference and not at an optically active surface.
In order to improve positional stability, the use of a front-screw ring, which is likewise provided with ribs, or a plug-in ring which is to be cemented in and which additionally fixes the lens in axial direction is proposed.
U.S. Pat. No. 7,471,470 B2 discloses a lens mount in which the lens is held radially via radially preloaded retaining arms (referred to therein as bracing struts) extending in axial direction of the lens mount. Axially, the lens rests by an end face on an annular collar which is formed at the same mount part as the retaining arms. During different thermal expansions between the lens and the lens mount, a relative movement is brought about compulsorily between the lens and the annular collar. To prevent this relative movement from leading uncontrolledly to a decentering of the lens, evenly distributed radial forces which keep the lens centered act via the retaining arms through the permanent preloading. However, these retaining forces also lead compulsorily to stresses in the lens and, therefore, to an impairment of the optical imaging quality of the lens, which cannot be tolerated in many applications.
A lens carrier is known from EP 1 094 348 B1, cited above, in which an annular groove is formed at the circumferential surface of a lens and radially elastic segments are formed monolithically at the lens mount, the free ends of which radially elastic segments engage radially in the annular groove, and the lens is held radially and axially exclusively via this connection. The geometry of the segments can vary and can accordingly be adapted to the existing space conditions for the lens mount in conformity with the material constants of the mount, the dimensioning of the segments and the required spring stiffness. The segments are identically dimensioned and, in a radially uniformly distributed manner, extend into the annular groove arranged at the circumferential surface such that the lens centers itself in the lens mount. The spring stiffness of the individual segments and the preloading force acting on the lens are equal.
A differing thermal expansion of the lens mount and lens is compensated in radial direction via the spring action of the segments. Dynamic loads are only transmitted to the lens in a damped manner in that the lens is connected to the mount exclusively via the elastic segments.
However, there are disadvantages to the lens being held in the lens mount substantially by frictional engagement. For one, the lens is held in an overdetermined manner by the plurality of segments extending into the annular groove, which requires a high-precision fabrication of the joint, i.e., of the annular groove and segments, within very close tolerances in order to prevent undefined tensions. For another, tensions in the lens as a consequence of the clamping forces which act on the held element and which are required for securely holding the lens even under mechanical loads are inevitable. While it may be attempted to put the annular groove in a plane in which force input has, if possible, only a slight effect on the optically active surfaces, an effect on imaging quality cannot be entirely prevented.
It is the object of the invention to improve a lens carrier according to EP 1 094 348 B1, cited above, such that less exacting demands can be made with respect to the manufacturing tolerances of the lens carrier and no retaining forces which are required for a frictional engagement act on the lens.
This object is met for a mount assembly with a monolithic mount and an element having an end face and at least one cylindrical circumferential surface, wherein the mount comprises a mount ring with an axis of symmetry at which is formed a plurality of retaining arms with free ends, which retaining arms are arranged concentrically around the axis of symmetry and extend at least partially in axial direction, and the free ends contact the element. Three of the retaining arms have a first length and contact the end face. The other retaining arms have a second length and are bonded to one of the at least one circumferential surface. The first length and second length are unequal.
The element advantageously has an outer circumferential surface, a circular end face bounded by the outer circumferential surface and a recessed circumferential surface adjoining the circular end face. The first length is greater than the second length and the retaining arms with the second length, which are accordingly shorter, are connected to the recessed circumferential surface, while the longer retaining arms with the first length are connected to the end face surrounding them. In this case, with the exception of their length, the retaining arms can be constructed identically with respect to geometry and dimensions.
The bonding connection of the retaining elements to the circumferential surface is advantageously produced by means of adhesive.
Recesses are advantageously formed at the free ends for receiving the adhesive.